1. Field of the Invention
This invention relates to a crystal growing process and more particularly to a process for growing single crystals of beryllium oxide from high temperature molten mixtures.
2. Description of the Prior Art
Beryllium oxide (BeO) is one of the most refractory, strong, and chemically inert oxides. However, because it is highly toxic, its use has been limited to applications which require its unique properties. Among the properties which have been exploited are its exceptionally high thermal conductivity combined with high electrical resistivity.
Initial interest in the growth of single crystal beryllium oxide resulted from the need to obtain pure samples on which to make more exact physical properties measurements. It was also demonstrated that single crystal films of silicon and gallium arsenide could be grown epitaxially on selected planes of BeO. Semiconductor devices based on silicon grown epitaxially on sapphire are commercially available, and experimental microwave devices based on silicon grown on BeO have been demonstrated. The higher thermal conductivity of BeO compared with silicon, gallium arsenide, and sapphire provides advantages for BeO in high output power devices. Single crystals of BeO are potentially useful as substrates for silicon and gallium arsenide single crystal films and as windows for high power ultraviolet lasers. Such applications require crystals which are completely free of inclusions, bubbles, and similar defects and which are about 1.5 cm to 2.5 cm diameter and at least 1 cm long. Such crystals are not available commercially. Furthermore, growth rates by existing methods are so slow as to severely restrict or preclude commercial use of these methods.
Thus, there is a need to prepare BeO single crystals with a high degree of perfection, to prepare the crystals rapidly and to prepare crystals large enough to serve as substrates for semiconductor single crystal films and as ultraviolet laser windows.
Preparation of single crystals of BeO from melts containing BeO and calcium oxide (CaO) is disclosed in U.S. Pat. No. 3,768,983, issued Oct. 30, 1973 to Elkins et al. However, that patent fails to disclose the composition of the atmosphere over the melt, induction heating frequency, seed crystal orientation and seed rotation and pull rates, which have been found to be critical in the growth of defect-free single crystals of BeO.